Visibility levels allow a single Plato web to be viewed by different
groups of people, who can see differing subsets of the items on that
web. They are conceptually a hierarchic set of layers.

How many visibility levels are there?

There are a maximum of 4 visibility levels: a public level and 3 protected
levels for which a username and password is required for authentication
and hence the appropriate entitlements. There must be at least one protected
level, in order to be able to create items on the web.

How do I reference these levels?

The levels are numbered 40 (public), 30, 20 and 10 respectively. Level
10 has the most privilege. By default they are named "Public",
"All member", "Forum", "Working Group",
but these tags can be changed.

What can each level do?

The public level has read-only access. The others may have read and
write access - but by default only level 10 has this facility. When
items such as documents are created they are given a visibility level.
All users at that level (or less) can see these items. For instance,
a document created at "All member" level (30) can be seen
by levels 30, 20 and 10, but not 40 - the public level. Similarly an
event created at level 10 can be seen only by that level.

How are visibility levels defined?

The Plato web designer first identifies if a public level is needed
and then how many protected levels. For each protected level one or
more mailing lists are identified, membership of any of which entitles
access at that level.

How does a user view a particular level ?

When a user logins in to gain access to a protected level, the system
evaluates the lowest level they may access and automatically sets this
as their view of the site.